This invention relates to sheet resistance heaters, and more particularly to a flexible continuous sheet heater of the type described in U.S. Pat. No. 4,485,297 issued Nov. 27, 1984, U.S. Pat. No. 4,523,085 issued June 11, 1985, U.S. Pat. No. 4,542,285 issued Sept. 17 1985 and U.S. Pat. No. 4,690,347 issued Sept. 1, 1987 all of which are incorporated herein by reference.
In general, such heaters include a paper or plastic substrate on which is printed a semiconductor pattern (typically a colloidal graphite ink) having (a) a pair of conductor contact pistons extending parallel to each other and (b) a heating portion (typically a plurality of transverse bars) extending between and electrically connected to the conductor contact portions. A metallic conductor (typically copper stripping) overlies each of the conductor contact portions, and an overlying sealing layer is bonded to the substrate closely adjacent the opposite edges of the conductor and holds the conductor in tight face-to-face engagement therewith with the underlying conductor contact pistons.
Typical uses include area (e.g., ceiling or floor) heaters, pizza box heaters, thin heaters for pipes, wide heaters for under desks and tables, spaced heaters for greenhouse plant use, and as military thermal signature targets.
In resistance heating devices of the type described in the aforementioned patents and patent applications, the heating portion typically comprises a plurality of regularly spaced bars of essentially uniform width. It has been found that, in some applications, it is difficult to avoid excess localized heating of the portion of the conductor contact portion adjacent the end of each bar, particularly when relatively high resistance ink is used. It is also difficult to optimize the width of the space between adjacent bars, if the space is too wide there may be zones of uneven heating between adjacent bars; if it is too narrow, there is insufficient space for consistently secure tie-down of the copper conductors.